Thermoablation consists of destroying target tissues by inducing a temperature increase of the cells above an irreversible damage threshold. This threshold is linked to exposure time at a set temperature; in the case of temperatures comprised between 50° C. and 60° C. the time is of a few minutes whereas from 60° C. upwards cell death is almost instantaneous. The temperature increase is obtained by dispensing energy into the target tissues by more or less invasive applicators. The forms of energy that are commonly used for thermoablation comprise mechanical waves, radiofrequency currents, infrared radiations, microwaves.
One of the most promising forms of energy for thermoablation is currently microwave energy, which provides an excellent compromise between efficiency of transfer of energy and depth of penetration in biological tissues. The delivery of microwave energy to tissues destined for thermoablation occurs by inserting percutaneously, endoscopically, laparotomically or laparoscopically interstitial applicators consisting of a coaxial antenna comprising an internal conductor, a dielectric layer that covers the entire length of the internal conductor, an external conductor that covers coaxially the dielectric layer and the internal conductor, except for a distal end portion of the latter, constituting the radiant end of the antenna. The design of antennas for thermoablation must take into consideration certain constructional requirements linked to the use that is made thereof, in particular: biocompatibility, great mechanical resistance, spheroid coagulative necrosis, an antenna diameter that is as small as possible.
In order to ensure spheroid coagulative necrosis, the antenna needs both a radiation figure that is spheroid and a cooling system for dissipating the heat generated by the supply line of the antenna. At the operating frequencies of a microwave thermoablation system, the transit of power through the coaxial cable is characterised by great attenuation matched by heating of the coaxial cable. The generated heat could cause necrosis of the tissues in contact with the external stem of the antenna over the entire length thereof. The presence of a cooling circuit of the supply line enables heat to be removed and thus enables the eccentricity of the necrosis to be reduced.
A problem that is common to many antenna designs for microwave thermoablation is the elongation of the radiation figure along the supply line of the antenna, with resulting low sphericity. This elongation can be avoided by different improvements to the antenna project. One of the most common ways of maintaining good containment of the radiation figure is to use a device, called an electromagnetic choke, or more briefly choke, which makes a quarter-wave impedance transformer terminating in a short circuit. The choke is physically a coaxial line consisting of a cylindrical conductor that coaxially surrounds the external conductor of the antenna and it is closed thereupon in a short circuit at its proximal end, whereas it is open at its distal end. The terms “distal” and “proximal” refer to the ends of the device, or a part or component thereof, facing respectively in the direction of the tip of the antenna, or in the opposite direction.
Between the cylindrical conductor and the external conductor of the antenna, one or more sleeves made of dielectric material are interposed, to fill the entire length of the choke. The length of the choke is equal to an odd number (usually one) of quarters of the wavelength in said dielectric of the microwaves emitted by the antenna. Lengths that are different from a quarter of the wave confer on the choke less than optimum properties but are nevertheless useful for the purpose of obtaining a proximal containment, and thus pronounced sphericity of the radiation figure of the antenna. The choke is usually obtained by inserting, around the dielectric surrounding the external conductor of the antenna, a metal cylinder with an internal diameter that is equal to the external diameter of the dielectric and of a length that is such as to make an electric length that is equivalent to what has just been disclosed. The end of the metal cylinder that is furthest away from the radiant end of the antenna is short-circuited on the external conductor of the antenna, completing the structure of the choke.
A microwave device for tissue ablation of the previously mentioned type is disclosed in Italian patent for industrial invention 0001361771 in the name of the applicant.
The microwave devices for tissue ablation known from the prior art are further provided with a penetrating tip connected electrically to the internal conductor of the antenna, the function of which is to facilitate the introduction of the antenna into the tissues of a patient.
The penetrating tip has to be connected to the body of the antenna with a connection with great mechanical resistance that ensures that the tip cannot become detached from the body of the antenna because of mechanical stress that acts on the antenna during penetration and extraction of the antenna through the tissues of a patient.
Normally, the penetrating tip is connected to the body of the antenna by welding. This type of connection does not, however, ensure great resistance of the tip, which, if it is subject to great stress, may detach from the body of the antenna and remain inside the body of a patient, with all the problems that this entails.